MESSRS. J. MILNE AtfD T. GRAY ON SEISMIC EXPERIMENTS. 881 
The most interesting point about the diagram is, perhaps, the evidence 
of a cycle which is passed through in about five vibrations. Corresponding 
vibrations are similarly numbered. 
Fig. 5. (See fifth set of experiments, fall 17.) Fig. 5 is a representation, magnified 
two and a-half times, of the record taken by two horizontal lever seismo¬ 
graphs at the 100 feet station for the 17th fall of the ball. 
The line A represents the record of transverse vibrations, and B that of 
the direct. These two sets of vibrations were recorded simultaneously on 
the same plate. The direct vibrations are apparently more rapid than the 
transverse ones. 
The chief point to be observed about this diagram is that the normal 
vibrations commence a short time before the transverse ones. 
It will be observed that although the normal vibrations are at first 
greater in amplitude than the transversal ones, they die out more quickly 
than the latter. 
Corresponding points in time are joined by cross lines. 
This record may be taken as characteristic of other records taken in the 
same manner. 
Fig. 6. (See seventh set of experiments, fall 36.) This diagram shows a magnified 
record of the normal vibrations on the line A, and vertical vibrations on 
the line B, as taken simultaneously at the 100 feet station. It will be 
observed that the vertical motion like the transverse motion is a little 
behind the direct motion. This we should anticipate, the vertical being 
like the transverse, a wave of distortion. 
Fig. 7. (See sixth set of experiments, fall 33.) This diagram illustrates one of the 
methods adapted for the determination of the velocity of propagation. In 
this method a circuit-closer and seismograph were used (see eighth set of 
experiments). 
The diagram was drawn at the 250 feet station, and as shown on this 
paper has been magnified two and a-half times. 
The upper line was drawn by a point on the end of the lever which 
could be deflected by an electromagnet, which magnet was in electric 
connexion with the circuit-closer placed near the falling ball. 
The lower line shows the diagram drawn by a horizontal lever seismo¬ 
graph, arranged to record normal vibrations. The portions a b were drawn 
before the circuit was closed by the fall of the ball; when the circuit was 
closed near to the ball the pointer of the electromagnet was deflected to c 
and held deflected. 
The plate travelled the interval represented by b'd' before vibrations 
commenced to be recorded at the 250 feet station. 
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